I think it would be a good idea to make most...if not all costume items and skins available through drops. More than likely the drop rates of such items would be rare, but the items can be proportional to dungeons/maps/bosses. I am aware this may have some economical drawbacks but as I said...the drop rate could be made relatively low. Hopefully not too too low where they virtually never drop but low enough to keep farmers modest. Let me know what you think.